15 décembre 2010

Thermal variation of the magnetic entropy in CdEr2Se4 and arrangement of the erbium magnetic atoms positions in the crystal structure.

We have just discovered a new spin ice, i.e., a magnetic system with a residual entropy at low temperature.


Statistical physics teaches us that a system’s entropy goes to a finite limit when the temperature approaches absolute zero. This is Nernst's principle, also called the third principle of thermodynamics. In a large majority of cases, this entropy limit is zero: the ground state of the system is unique. The energy of any other state is larger. Exceptions to this rule exist, however. For instance: ice. In its most common form, a certain degree of disorder is permitted among the proton locations: the system can adopt a large number of states which are distinct but share the same energy. The system is degenerate: this leads to a residual entropy.


A magnetic analog of ice, the so-called spin ice, was discovered at the end of the 1990s in a family of mixed oxides of rare earths and non-magnetic metals. The magnetic moments form a lattice of corner-sharing tetrahedra. In this  structure, named after the pyrochlore mineral, it is not possible to simultaneously minimise all of the magnetic interactive energies. This is the magnetic frustration phenomenon, which gives rise to very exotic behaviours at low temperature and which are not well understood.


In collaboration with Spanish colleagues, we have found this behaviour in a system of spinal structure for the first time. Among the spin ice signatures, we found the existence of residual entropy (Fig.). This discovery opens up new avenues for the determination of the relationship between the balance of the different interactions and the nature of the ground states.  More generally, it allows for better insight into these new states of matter which are observed in these frustrated magnets.


Further reading: J. Lago et al., Phys. Rev. Lett. 104 (2010) 247203         


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